I install residential and commercial solar. Sometimes it gets hot when working on the roof and I wear a hard hat to protect me from the sun. The hard hat has lots of room around and above your head to allow for air to flow so it keeps me cooler than a regular hat. Having a fan that circulates air inside the hat helps to keep cooler air flowing over my head.
Step 1: Find the Materials Needed
I bought my materials from JK Electronics in Westminster, CA but I'm sure Radio Shack or a local electronics store would carry these items.
12 volt squirrel cage type fan - $5.00 (will run on any voltage from 3 volts up to 12 volts, more volts = more speed)
9 volt battery housing with switch - $2.89
9 volt battery - $1.89
18-20 gauge crimp connectors (2) - $0.25
Self adhering cable tie blocks (2) - $0.50
I got the white full brim hard hat from The Home Depot for about $14.00
Step 2: Attach Fan and Battery to Hard Hat
I used hot melt glue to attach the fan to the hard hat. Make sure you don't get glue into the moving parts of the fan. For the battery enclosure, I used self adhering cable tie blocks. The battery enclosure that I bought has the battery door on one side and the switch on the other. I need a way to replace the battery and gluing the enclosure would have prevented this. After everything is mounted, I cut the wires to length and crimped them together.
I have a sun shade that shades the back of my neck but still allows air to flow. A wet bandana or a Buff around your neck will also help. Stay Cool!
Update 7.1.14: After using this for a couple of weeks I can say that it does keep my head cool. I shave what's left of my hair so the air is blowing directly on my head. If you have thick, luscious, beautiful hair, this might not work as well for you. I'm going through one 9 volt battery about every 15 hours (which is one and a half work days for me). I'm going to research using a small solar panel, rechargeable battery and charge controller (maybe Arduino).